1. Field
The present invention relates generally to communication, and more specifically to a novel and improved method and apparatus for call setup latency reduction in a wireless communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards. These named standards are incorporated herein by reference.
Call setup is the process of establishing dedicated physical channels and negotiating service configuration parameters between a mobile station and a base station so that communication can take place. Call setup procedures fall into two classes. Mobile station originated call setup occurs when a mobile station user makes a call. Mobile station terminated call setup occurs when a call is made to the mobile station.
Call setup procedures involve signaling between a mobile switching center (MSC) or packet data service node (PDSN), one or more base stations (BS), and a mobile station (MS). As used herein, the term base station can be used interchangeably with the term access point. The term mobile station can be used interchangeably with the terms subscriber unit, subscriber station, access terminal, remote terminal, or other corresponding terms known in the art. The term mobile station encompasses fixed wireless applications. Signals from the mobile station are known as the reverse link, reverse channel, or reverse traffic. Signals to the mobile station are known as the forward link, forward channel, or forward traffic.
FIG. 1 depicts a mobile station originated call setup procedure as defined in Release A of the cdma2000 standard. In step 1, the mobile station sends an Origination Message 1 to the base station. This message indicates to the network that the mobile station user wants to make a call. It contains dialed digits and a service option number to indicate the type of call (i.e. voice, data, etc.). A list of pilot signals from neighboring base stations that have been received at the mobile station with sufficient strength are also included in this message, so that the base station can determine which pilots to include in the active set.
In step 2, upon successfully receiving the Origination Message 1, the base station sends a Base Station Acknowledgement Order 2 to the mobile station. This message acknowledges the receipt of the Origination Message 1.
In step 3, the base station sends a Connection Management (CM) Service Request Message 3 to the MSC, which triggers the MSC to setup the call. This message contains relevant information received from the mobile station in the Origination Message 1.
The MSC responds with an Assignment Request Message 4 to the base station in step 4. This message indicates to the base station to setup the radio channel. However, the base station has the option of setting up the radio channel as soon as the Origination Message 1 is received.
Note that in this figure, as well as in those figures described below, the order in which the Assignment Request Message 4 is delivered from the MSC to the base station in relation to other message deliveries is somewhat flexible. There are rules limiting that flexibility. The Assignment Request Message 4 will be sent from the MSC to the base station after the MSC receives the CM Service Request Message 3 (for mobile station originated call setup) or the Paging Response Message 25 (for mobile station terminated call setup, described below). The Assignment Request Message 4 comes before the base station sends the Service Connect Message 10 to the mobile station, described below.
In step 5, the base station sends a Channel Assignment Message 5 to the mobile station. The standard also defines an Extended Channel Assignment Message. As defined herein, the Channel Assignment Message 5 represents either message. This message assigns a dedicated physical channel to the mobile station for the purpose of carrying the user traffic associated with the call. It includes the relevant information for all pilots in the active set of the mobile station. After this step, the mobile station enters the traffic state 450. A state diagram including that state and others is detailed below with reference to FIG. 4.
In step 6, upon receiving the Channel Assignment Message 6, and after receiving two consecutive good frames on the forward link, the mobile station sends a preamble to the base station to help the base station acquire the reverse link signals from the mobile station. Once the reverse link has been acquired, the base station sends the Base Station Acknowledgement Order 7 to the mobile station in step 7. Upon receiving the Base Station Acknowledgement Order 7, the mobile station sends the Mobile Station Acknowledgement Order 8 to the base station in step 8 to indicate that the mobile station has acquired the forward link being transmitted by the base station.
Now the dedicated physical channels have been successfully set up. In step 9, a service negotiation procedure takes place between the mobile station and the base station to determine the format of information transfer. Examples of negotiated items include frame rate, frame type, transmission rates, and type of traffic (i.e. voice or data, vocoder rate if applicable). Some items are specified by the base station and therefore non-negotiable (e.g. mapping of logical channels to physical channels). Negotiation may involve multiple exchanges of Service Request Messages and Service Response Messages between the mobile station and the base station. The information exchanged is contained in a Service Configuration information record. The final negotiation message sent, in step 10, is a Service Connect Message 10 from the base station to the mobile station. Both the Service Configuration information record and a Non-Negotiable Service Configuration information record are sent. The standard also allows the General Handoff Direction Message or the Universal Handoff Direction Message to be sent instead of the Service Connect Message in situations where a radio handoff becomes necessary while service negotiation is in progress.
In some instances service negotiation, step 9, can be avoided. If the mobile station is to use a previously stored service configuration, the base station simply sends a Service Connect Message 10, step 10, with an indication to use the previously stored service configuration. In the standard, this corresponds to setting the USE_OLD_SERV_CONFIG flag to ‘01’.
In step 11, upon receiving the Service Connect Message 10, the mobile station sends a Service Connect Completion Message 11 to the base station to indicate that it has accepted the proposed service configuration. Upon receiving the Service Connect Completion Message 11, in step 12, the base station sends an Assignment Complete Message 12 to the MSC to indicate that the base station has successfully set up the call.
After step 10, the Service Connect Message 10, the service configuration specified by the message takes effect. Now the call setup is complete and user traffic (i.e. voice or data) between the mobile station and the base station can flow. The traffic will flow between the base station and the MSC (for voice calls) or between the base station and the PDSN (for packet data calls) after step 12, the Assignment Complete Message 12.
FIG. 2 depicts a mobile station terminated call setup procedure as defined in Release A of the cdma2000 standard. First, the MSC sends a Paging Request Message 21 to the base station to indicate a call is incoming to the mobile station. Second, a General Page Message 22 is sent from the base station to the mobile station. The standard also identifies a Universal Page Message, whose function is similar to the General Page Message 22, and the latter term will be used throughout to indicate either message. This message may be sent out over one or more sectors. This message indicates to the mobile station that it is receiving a call, and the Service Option number corresponding to the call.
Third, upon receiving the General Page Message 22, the mobile station sends a Page Response Message 23 to the base station, including the list of pilots, similar to that described in Origination Message 1 above, so that the base station can determine the appropriate active set. Fourth, upon successfully receiving the Page Response Message 23, the BS sends a Base Station Acknowledgment Order 2 to the mobile station, as described in step 2 with respect to FIG. 1 above. This message acknowledges receipt of the Page Response Message 23.
Fifth, the base station sends a Paging Response Message 25 to the MSC, which triggers the MSC to set up the call. The subsequent steps shown in FIG. 2 correspond to the like-numbered steps and messages described in steps 4 through 12 above with respect to FIG. 1.
Each step in the call setup procedures just described contributes to the call setup latency. Call setup latency, or the time required to set up a call, is an increasingly important parameter in wireless system design as data use becomes more prevalent. Modern wireless data communication systems offer “always on” connectivity. As those skilled in packet-switched network design know, “always on” connectivity does not mean a physical channel is permanently dedicated to a specific user. This would be bandwidth inefficient, and unlikely to be cost-effective for subscribers. Instead, when a mobile station engages in data communications, a call is setup to allow one or more packets to be transmitted, then the call is torn down to free up the channel for another user. In a typical data communication session, calls will be set up and torn down repeatedly, with call setup latency occurring during each call. Naturally, decreasing call latency, while important in voice communications as well, is very important to provide a compelling user experience to the wireless data user.
Each step, described above, introduces latency due in part to the time required to transmit each message, and in part due to the processing time required to receive each message and determine the appropriate next step. Furthermore, much of the call setup signaling occurs on common channels which are shared by a number of mobile stations and a base station. As such, a component of the call setup latency is introduced when a mobile station must make repeated attempts to gain access to the common channel (known as the access channel). Furthermore, a message for a particular mobile station may have to be queued with messages for other mobile stations, yet another source of latency in performing the steps described above. Therefore, reducing the number of steps in the call setup procedure is one effective means to reduce call latency, as is reducing the transmission and/or processing time associated with any remaining messages.
One example of a reduced-latency call setup procedure is defined in the HDR specification, and depicted in FIG. 3. Such a system is disclosed in U.S. patent application Ser. No. 09/707,569, entitled “METHOD AND APPARATUS FOR ADAPTIVE TRANSMISSION CONTROL IN A HIGH DATA RATE COMMUNICATION SYSTEM”, filed Nov. 6, 2000, assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 3 depicts a mobile station terminated call setup procedure with reduced steps compared to the procedure described with respect to FIG. 2. Essentially, steps 2 through 4, corresponding to messages 22, 23, and 2 in FIG. 2 respectively, are removed. Instead of the base station sending the mobile station a General Page Message 22 in response to the Paging Request Message 21 from the MSC, the base station sends a modified Channel Assignment Message 30. Channel Assignment Message 30 takes the place of the General Page Message 22 (step 2 in FIG. 2) and Channel Assignment Message 5 (step 7 in FIG. 2). This eliminates the need for the Page Response Message 23 (step 3 in FIG. 2) and the Base Station Acknowledgement Order 2 (step 4 in FIG. 2). The removal of these three steps significantly lowers call setup latency.
The steps of the procedure of FIG. 3 are as follows. First, the MSC sends the base station Paging Request Message 21. In response, the base station sends the mobile station identified in Paging Request Message 21 a Channel Assignment Message 30, as just described. The mobile station enters the traffic state 450 after receiving this message. After receiving two consecutive good frames on the forward link, the mobile station sends a preamble 6 to the base station. The base station acknowledges acquisition of the preamble 6 by sending the mobile station a Base Station Acknowledgement Order 7. In response, the mobile station sends the base station a Mobile Station Acknowledgement Order 8. The base station sends the MSC a Paging Response Message 25 to trigger the MSC to set up the call. Assignment Request Message 4 is delivered from the MSC to the base station. Service negotiation 9 then takes place, unless a mitigated by an indication to use a previously stored service configuration (i.e. setting USE_OLD_SERV_CONFIG to ‘01’). Service Connect Message 10 is delivered from the base station to the mobile station to end any negotiation. The mobile station accepts the Service Connect Message 10 with a Service Connect Completion Message 11. The base station lets the MSC know that the call is set up with an Assignment Complete Message 12.
After the Service Connect Message 10, the service configuration specified by the message takes effect. Now the call setup is complete and user traffic (i.e. voice or data) between the mobile station and the base station can flow. As described above with respect to FIG. 1, the traffic will also flow between the base station and the MSC (for voice calls) or between the base station and the PDSN (for packet data calls) after step 12, the Assignment Complete Message 12.
FIG. 4 depicts a mobile station state diagram. The states shown are general states useful for describing call setup, and do not represent every state a mobile station can enter. Furthermore, not all possible state transitions are shown. Rather, the subset useful for discussing the various aspects of the present invention is shown. State 410 is a power up state, the state a mobile station enters when it is powered on. The mobile station then proceeds to the initialization state 420, in which the mobile station attempts to acquire a system. Once system timing for at least one base station is acquired, the mobile station enters the idle state 430, where it monitors the paging channel for any messages directed to it, such as General Page Message 22 or Channel Assignment Message 30, described above.
From the idle state 430, the mobile station may enter the system access state 440 for a number of reasons. The system access state is entered when the mobile station wishes to communicate on the access channel (shared among a plurality of mobile stations) to a base station. One reason for entering the system access state and communicating on the access channel is when a mobile station has entered a new cell boundary or recently powered up and needs to register its location with a base station. Another reason is to respond to a General Page Message 22 or Channel Assignment Message 30, described above (for mobile terminated calls). A third reason is for sending an Origination Message 1, described above (for mobile originated calls). If a call setup procedure is initiated, such as those described above, the mobile station proceeds to the traffic state 450 upon successful call setup. This state was referenced in FIGS. 1-3, above.
The mobile station leaves system access state 440 to reenter idle state 430 when a registration is complete (and no call setup was initiated), a message is completed that does not require the mobile station to remain in the access state, the mobile station fails to gain access on the common access channel (for reasons including congestion due to other mobile stations' accesses), or when the base station fails to acknowledge a transmitted message. Furthermore, failure to gain access or failure to receive acknowledgement may cause the mobile station to revert to the initialization state 420, depending on how the system is designed. It may be that upon these failure events, it is advisable to attempt to acquire a different base station rather than to make additional attempts with a base station that is not responding.
Idle state 430 is left for initialization state 420 when the mobile station is unable to receive pages (meaning a new base station may need to be acquired), or the mobile station is directed to perform an idle handoff (that is, directed to cease monitoring the common channel of the current base station and acquire the common channel of a neighboring base station instead).
Useful in a wireless communication system is a short data burst (SDB) feature. This allows a small packet of information to be encapsulated in a message from a mobile station to a base station on the access channel. Therefore, a complete call setup is not required, since the traffic state is never entered. Such a SDB feature is specified in cdma2000. The SDB procedure is carried out as follows. From the system access state, a mobile station sends a Data Burst Message to the base station which includes the SDB information packet. The base station sends an Application Data Delivery Service (ADDS) Transfer Message to the MSC, which includes the SDB information packet as well as application layer information (i.e. identifying the type of packet, such as SDB, short messaging service (SMS), position location, and the like). The base station acknowledges the Data Burst Message by sending a Base Station Acknowledgement Order to the mobile station. The MSC (or PDSN) routes the packet data accordingly.
One example of the use of SDB is when an Internet Protocol (IP) packet is encapsulated in the SDB information. In this case, the MSC or PDSN can route the packet to a destination on the Internet or an intranet, perhaps to an application server. In some instances, an SDB packet delivered to an application server may serve to initiate data communication between the server and the mobile station which may ultimately require a traffic channel to be set up for the continued communication. Under these circumstances, the SDB message will be followed by a complete call setup procedure such as that described in reference to FIG. 1. And, as mentioned previously, the ongoing communication between the application server and the mobile station may entail numerous call setups, a byproduct of the nature of packet data communications. This example serves to further highlight the need for minimizing call setup latency.
As described, call setup latency is formed through multiple message transmissions and corresponding acknowledgements, the length of each message, and the associated processing required with each message. Call setup latency is one cause of delay that is undesirable in many communication applications: voice communications as well as data communications. To the extent that multiple calls must be setup during a communication session, a typical scenario with data, the delay introduced is exacerbated. There is therefore a need in the art for communication systems that minimize call setup latency.